Polytetrafluoroethylene coating compositions containing an alkali metal silicate and colloidal silica and articles coated therewith



United States Patent 6 POLYIETRAFLUOROETHYLENE COATING COM- POSITIONSCGNTAINING AN ALKALI METAL SILICATE AND COLLOIDAL SILICA AND ARTI- CLESCOATED THEREWITH James Robert Huntsberger, Wynnewood, Pa, assignor to E.I. du Pont de Nemours and Company,-Wilrnington, Del., a corporation ofDelaware No Drawing. Application January 21, 1954 Serial No. 405,507

15 Claims. (Cl. 117-232) This invention relates to aqueouspolytetrafiuoroethylone coating compositions and more particularly topolytetrailuoroethylene coating compositions having improved adhesion tometals, especially copper.

A customary and convenient form of aqueous polytetrafluoroethylenecoating composition is an aqueous dispersion of colloidalpolytetrafiuoroethylene as disclosed in U. S. Patents 2,478,229 and2,559,752 to K. L. Berry. The products of these inventions are commonlyreferred to as aqueous polytetrafiuoroethylene suspensoids.

The inertness, insolubility, and lubricity or non-adhesivecharacteristics of polytetrafiuoroethylene coatings are attributes whichmake these polymers useful for many purposes for which most organicfilm-forming polymers are inadequate. One serious deficiency ofpolytetrafiuoroethylene coatings is their poor adhesio n to metal, i.we. the coalesced film can be rather easily removed from the substrate.

It is a general object of this invention to improve the adhesion ofpolytetrafiuoroethylene coatings to metal substrates, particularlycopper and copperrbearing alloys,

vwithout adversely aifecting other essential properties such asinsensitivity to water.

A specific object is to provide aqueous polytetrafluoroethylenecoatingcompositions which yield coatings having improved adhesion tometal substrates, particularly copper or copper-bearing alloys. 7

A further .objectis to provide metal articles having.polytetrafiuoroethylene surface coatings characterized by improvedadhesion.

A still further object is to provide copper electrical conductorswith,polytetrafiuoroethylene insulation having high abrasion resistance.

Other objectswill become apparent as the description .of the inventionproceeds.

These objects are accomplished by providing coating compositionscomprising an aqueous suspensoid .of colloidal polytetrafluoroethylene,water-soluble alkali metal silicate and colloidal silica.

More specifically, these objects are accomplished by providing aqueouscoating compositions comprising water, colloidalpolytetrafiuoroethylene, and l%-85% by weight based on thepolytetrafluoroethylene of watersoluble alkali metal silicate togetherwith colloidal silica, the ratio of said silicate to said silica beingbetween 25:75 and 90:lO, by weight.

Further improvements are obtained by adding to the compositions definedimmediately above a water-soluble alkali metalalkyl siliconate.

The protective coatings and coated articles of this invention areprovided by applying a coating of an above defined coating compositionto a suitable substrate and baking-the coating at a temperaturesuflicient to coalesce the polytetrafluoroethylene particles and producea continuous adherent film, e. g. at about 620 F. or higher.

"Aqueous coating compositions containing colloidalpolytetrafluoroethylene and water-soluble alkali metal silicate areclaimed in the copending application of Jerome Hoch- 2,825,664 PatentedMar. 4, v1958 ice .metal silicates having an SiO molar proportion above4 can .be prepared and may be employed in this invention. Examples ofsuitable water-soiuble alkali metal silicates are potassium silicate andsodium silicate.

Aqueous dispersions of colloidal s lica are readily available for use inthis invention.

Examples of suitable alkali metal alkyl siliconates are DOW CorningXS-l, a water soluble sodium methyl siliconate, and G. E. SC-SO, a watersolution of sodium methyl siliconate. Such compositions may also bedescribed as alkali metal salts of short chain alkyl substitutedsiliconic acid.

The following examples of this invention are given by way ofillustration but not of limitation. The parts and percentages are byweight.

Example 1 A coating composition of this invention was prepared -by.intimately mixing the following ingredients:

Parts by weight The polytetrafiuoroethylene suspensoid containedcolloidal polytetrafluoroethylene, and 1.8% octyl phenyl polyglycolether, the balance being Water containing trace amounts of innocuousmaterials resulting from the preparation'of the suspensoid.

The potassium silicate solution contained 27% potassium silicate havinga molar ratio of SlO IK O of 3.9151.

The aqueous colloidal silica contained 28% colloidal silica, the balancebeing water containing a stabilizing amount of sodium hydroxide. i i iThe resultingcoating composition contained 31% ofpolytetrafluoroethylene based on the total Water andpolytetrafluoroethylene, and 30% of potassium silicate and 30% ofcolloidal silica based on the polytetrafluoroethylene, the silicate tosilica ratiobeing 50:50.

' One coat of this composition was sprayed on a copper panel and bakedfor 3 minutes at 750 F. Thereafter, three successive coats of aconventional polytetrafluoroethylene top coat composition of thefollowing formula were sprayed on the panel, each being baked for 5minutes at 750? F.

Parts by weight Aqueous polytetrafluoroethylene suspensoid (aspreviously described in this example) 78.3 Water 8.5 Sodium laurylsulfate dispersing agent 1.9 Octylphenyl polyglycol ether dispersingagent 1.3 Toluene n 10.0

bake at 750 F.

in Example 1.

siliconate.

Under these conditions, the instrument recorded a pull of 4.2 poundswhich is designated as the adhesion value of the coating to the metal.Under similar conditions of preparation and testing,apolytetrafiuoroethylene coating containing no silicate or silica had anadhesion value of less than 0.5 pound. A polytetrafluoroethylene coatingcontaining, in the first coat, 30% potassium silicate based on the.polytetrafluoroethylene, but no silica, had an adhesion value of 3.5pounds. It is seen that the addition i .of colloidal silica produced amarked increase in adhesion.

In similar tests it was determined that a polytetrafluoroethylenecoating containing, in the first coat, 12.5% potassium silicate and12.5% of colloidal silica based onfthepolytetrafluoroethylene had anadhesion value of 1.9 pounds in comparison'with less than 1 pound whenthe first coat differed only by not containing colloidal silica. Also, acoating containing 54% potassium silicate and 28% colloidal silica basedon the polytetrafluoroethylene had anfadhesion value of 4 pounds versus3 pounds for the same composition containing no colloidal silica.Example-2 A coating composition of this invention was prepared byintimatelymixing the following ingredients:

. a V Parts by weight Aqueous polytetrafluoroethylene suspensoid 48.0Aqueous sodium silicate solution 28.0 Aqueous colloidal silica 24.0 7

The polytetrafiuoroethylene suspensoid and colloidal silicia were'thesame as employed inExample 1.

The sodium'silicate'solution contained 37.6% sodium .Aqueouspolytetrafiuoroethylene' suspensoid 46.4 Aqueous potassium silicatesolution 20.8 Aqueous colloidal silica 18.5 Aqueous sodium methylsiliconate solution 9.2 Aqueous 20% sodium hydroxide solution 5.1

as described in Example 1, the product of this example had an adhesionvalue greater than 7 pounds, at which point the film broke as it. wasbeing stripped from the panel.

Example 4 V The product of Example 2 was modified by adding to 100 partsthereof 5 parts of an aqueous 31.2% sodium methyl siliconate solution,amounting to 3.4% of'the siliconate based on the totalpolytetrafluorethylene, silicote and silica.

when tested as in Example 2, the resulting coating had an adhesion valueof 3.9 pounds.

Example .5 V A coating composition of this invention wasiprepared byadding and mixing the following'ingredients in the order named; s

Parts by weight The first four ingredients were the same as employed inExamp1e3.

The resulting composition contained 34.6% polytetra- 'fluoroethylenebased on the total water and polytetra- .fiuoroethylene, 20% potassiumsilicate and 18.7%' 001- loidal silica based on thepolytetrafluoroethylene, and

" 7.5% sodium methyl siliconate based on the totalpolytetrafiuoroethylene, silicate and silica, the ratio of silicate tosilica being 52:48. a

4 This composition was used as aprimer or first c at on wire. It wasapplied by passing a single strand of #25 AWG bare copper wire through abath of the composition. at a rate of 10. 6 feet per minute, withdrawingthe wire vertically, and baking the coating thereon by passing the wirethrough a vertical oven four feet long at about 800 F. Then threeconsecutive coats of a polytetrafluo- 'roethylene wire enamel wereapplied and baked in the The resulting coating had an adhesion 'value of2.8 '7

pounds versus less than 2 pounds for asimilar composi 7 tion containingno colloidal silica. i

' Example 3 A coating composition of this invention was prepared byintimately mixing the following ingredients:

. V Parts by weight Aqueous polytetrafluoroethylene suspensoid' 49.40

Aqueous potassium silicate solution; 29.60

Aqueous colloidal silica e 19.76

Aqueous sodium methyl siliconate solution"... 1.24 H

The first three ingredients were the same as employed The siliconatesolution was G. 'E. SC-SO, supplied by General Electric Company Itcontained 31.2% of the Y The resulting composition contained 34.8%polytetra- 'fiuoroethylene based onrthe total water andpolytetrafluoroethylene, 27% potassium silicate and 18.7% col--- aloidal silica based on the .polytetrafluoroethylene, and

. 0.9% sodium methyl'siliconate based on the totalpolytetrafluoroethylene, silicate and to silica being59z4l. V

When appliedto a copper panel, topcoated with a silica, the ratio ofsilicate polytetrafluoroethylene top coat composition and tested samemanner. This polytetrafiuoroethylene wire enamel .contained 60%colloidal polytetrafluoroethylene and'3% sodium lauryl sulfatedispersing agent, the balance being water containing trace amounts ofinnocuous materials resulting from the preparation of the colloidalpoly- The resulting coating was about 1.5 mils thick. This insulatedwire had a dielectric breakdown voltage of 2600 volts per mil ofcoating. The insulation resistance was greater than 20,000 megohms. Thescrape abrasion as measured with the General Electric Sc'rapeAdhesion'lester was'154 strokesrunder a 200 gram 'load. 7

Wire similarly coated with polytetrafluoroethylene, the first coat of.Which'did not containasilic ate, silica or siliconate, had a dielectricstrength of 2100 volts per mil and the scrape abrasion was 63 strokesunder a .200 gram.

1oad."

It is to be understood thatthe particular ingredients employed. in theexamples'are merely illustrative and that, in particular, the'concentration of the essentialmate'rialin any ingredient may be'varieddepending upon its source. For instance, a 20% dispersion of colloidalsilica can be used'in place-of a 28% dispersion by making s'suitableadjustments in the other-ingredients to yield a -.tion of essentialingredients. kali metal silicate having an $0; to alkali metal oxideproduct of this invention having the specified concentraratioof 1:1, 2:1or 3.5:1 canbeused instead ofrthose employed in the exampleshavingratios of 3.9;1 and 3.33:1.4 i 7 Likewise the wetting or dispersingagents employed in the examples maybe replaced partly or wholly'by otherWell known surface-active agents.

Also, for. instance, an al Finely divided extender pigments or coloredpigments may be incorporated by conventional dispersion methods. Alsominor amounts of additives such as film-forming materials, volatileorganic liquids, acids and alkalis may be incorporated in thecompositions of this invention to further modify the storage,application or film properties.

Colloidal polytetrafluoroethylene may be employed in a wide range ofconcentrations, preferably %-70% by weight based on the total water andpolytetrafluoroethylene. Below 10% the compositions yield very thinfilms, several of which are required to build up a protective coating,the process being unnecessarily costly. Above 70% the compositions areviscous or thixotropic, making uniform application diflicult. Aparticularly preferred range of concentration of colloidalpolytetrafluoroethylene is %40% by weight based on the total water andpolytetrafluoroethylene, such compositions being easily applied andyielding relatively thick films.

Water-soluble alkali metal silicate together with colloidal silica, bothpreferably in the form of aqueous solutions or dispersions, may beemployed in total concentrations of 10%-85% by weight based on thepolytetrafluoroethylene. The preferred range is 15%-40%. The ratio ofalkali metal silicate to colloidal silica must be between :75 and 90:10, by weight, to produce markedly improved adhesion to metals withoutadversely affecting other essential properties such as resistance towater. The preferred range of alkali metal silicate to colloidal silicais between :50 and 80:20 by weight.

A water-soluble alkali metal alkyl siliconate, preferably in the form ofan aqueous solution, may be employed to further enhance adhesion or toreduce the amount of silicate and silica required to yield a givenadhesion value, preferably in the amount of 0.5 %20.0% by weight basedon the total polytetrafluoroethylene, alkali metal silicate and silica.The particularly preferred range is l%10%.

The examples show the most important advantages of the products of thisinvention, i. e. improved adhesion and electrical properties on copperand copper-bearing alloys. However, the products are not limited to suchuse, and they may be applied to ferrous metals, aluminum, ceramics,glass, asbestos and other heat resistant substrates in the form of solidrigid articles, porous articles, flexible articles and fabrics. While asingle coat in direct contact with the substrate, i. e. as a primer, isthe most advantageous method of using the compositions of thisinvention; obviously they may be used in more than one coat, with orwithout subsequent top coats of other coating compositions.

The compositions may be applied by any convenient method such asspraying or dipping, followed by baking at a temperature sufiicient tocoalesce the polytetrafluoroethylene particles into a continuous film.The resulting coatings are useful for protecting the substrate fromweather, abrasion or chemical agents, for insulating electricalconductors, or for coating molds, bread pans, cookie sheets or the likewith greaseless films to which the material being handled does notstick.

The compositions are most conveniently and economically prepared forwarehousing and shipment in relatively concentrated form. Suchcompositions are easily dilutable to application consistency with water.

It is apparent that many widely different embodiments of this inventioncan be made without departing from the spirit and scope thereof; and,therefore, it is not intended to be limited except as indicated in theappended claims.

I claim:

1. An aqueous coating composition comprising water, colloidalpolytetrafluoroethylene, and 10%-85% by weight based on thepolytetrafluoroethylene of watersoluble alkali metal silicate togetherwith colloidal silica, the ratio of said silicate to said silica beingbetween 25:75 and 90:10 by weight.

2. A coating composition of claim 1 comprising 15 %40% by Weight basedon the polytetrafluoroethylene of water-soluble alkali metal silicatetogether with colloidal silica.

3. A coating composition of claim 1 in which the ratio of said silicateto said silica is between 50:50 and :20.

4. A coating composition of claim 1 in which said silicate is potassiumsilicate.

5. A coating composition of claim 1 in which said silicate is sodiumsilicate.

6. A coating composition of claim 1 in which the colloidalpolytetrafluoroethylene is present in the amount of 10%-70% by weight ofthe total water and polytetrafiuoroethylene.

7. A coating composition of claim 1 in which the colloidalpolytetrafluoroethylene is present in the amount of 20%40% by weight ofthe total water and polytetrafiuoroethylene.

8. A coating composition of claim 1 also comprising sodium methylsiliconate.

9. A coating composition of claim 1 comprising 0.5%20% alkali metalalkyl siliconate, by weight based on the total polytetrafluoroethylene,silicate and silica.

10. A coating composition of claim 1 comprising 1%10% alkali metal alkylsiliconate, by weight based on the total polytetrafluoroethylene,silicate and silica.

11. An article having an adherent baked coating of a composition ofclaim 1.

12. An electrical conductor having a baked insulating coating of acomposition of claim 1.

13. A coating composition of claim 1 also comprising alkali metal alkylsiliconate.

14. An article having an adherent baked coating of a composition ofclaim 13.

15. An electrical conductor having a baked insulating coating of acomposition of claim 13.

References ited in the file of this patent UNITED STATES PATENTS2,386,674 Flint et al. Oct. 9, 1945 2,592,147 Ikeda Apr. 8, 19522,710,266 Hochberg June 7, 1955

1. AN AQUEOUS COATING COMPOSITION COMPRISING WATER, COLLODIALPOLYETRAFLUOROETHYLENE AND 10%-85% BY WEIGHT BASE ON THEPOLYTETRAFLUOROETHYLENE OF WATERSOLUBLE ALKALI METAL SILICATE TOGETHERWITH COLLOCIAL SILICA, THE RATIO OF SAID SILICATE TO SAID SILICA BEINGBETWEEN 25:75 AND 90:10 BY WEIGHT.